2025/08/19 by Natalie H. Allen, Allen, Natalie H., Néstor Espinoza +41 · 2 citations
Earth and Planetary Sciences · Physics and Astronomy · #Astro and Planetary Science #Earth and Planetary Astrophysics (astro-ph.EP) #FOS: Physical sciences #Geophysics and Gravity Measurements #Planetary Science and Exploration
paper · pdf · doi:10.48550/arxiv.2508.14210
openalex publication_date 2025/08/19 · openalex created_date 2025/10/16 · openalex updated_date 2026/07/28
It is an open question whether small planets around M dwarfs are able to maintain atmospheres. The Hot Rocks Survey aims to address this question by observing 9 rocky exoplanets orbiting M dwarfs with MIRI emission photometry to constrain the onset of atmospheres. In this paper, we present two MIRI F1500W (15μm) eclipses of LTT 3780 b, an ultra-short period super-Earth (P=0.768 d, R=1.325 R_⊕, M = 2.46 M_⊕) that receives 111x Earth's instellation, the highest in the survey. We find a combined eclipse depth of 312±38 ppm, which is consistent between different data reduction and analysis assumptions, bolstering our confidence in the eclipse detection. This eclipse depth is consistent with the thermal emission from a bare rock surface, with a dayside temperature of Td=1143+104-99 K, 98±9 % of the maximum temperature predicted for a zero albedo, zero heat redistribution blackbody. We are able to confidently rule out CO2-based atmospheres down to 0.01 bar surface pressure to greater than 3σ (ruling out an approximately Mars-like atmosphere). We are unable to rule out a pure H2O 1 bar atmosphere, though we argue that this composition is unlikely on such a highly irradiated planet, nor O2 atmospheres due to the lack of features in the bandpass, though we can put constraints on CO2-mixture atmospheres. As a potential bare rock, we consider a variety of surface composition models, but are unable to distinguish between them. However, LTT 3780 b is an excellent target for follow-up JWST observations to determine its surface composition and rule out additional atmospheric compositions.